SOS response in E. coli describes the coordinate induction of many enzymes, including repair activities, in response to irradiation or other damage to DNA; results from activation of protease activit by RecA to cleave LexA repressor.
Southern blotting describes the procedure for transferring denatured DNA from an agarose gel to a nitrocellulose filter where it can be by bridized with a complementary nucleic acid.
Spheroplast is a bacterial or yeast cell whose wall has been largely or entirely removed.
Spindle describes the reorganized structure of a eukaryotic cell passing though division; the nucleus has been dissolved and chromosomes are attached to the spindle by microtubules.
Splicing describes the removal of introns and joining of exons in RNA; thus introns are spliced out while exons are spliced together.
Splicing junctions are the sequences immediately surrounding the exon-intron boundaries.
Spontaneous mutations are those that occur in the absence of any added reagent to increase the mutation rate.
Sporulation is thegeneration of a spore by a bacterium (by morphological conversion) or by a yeast (as the product of meiosis).
SSB is the single-strand protein of E. coli, a protein that binds to single-stranded DNA.
Staggered cuts in duplex DNA are made when two strands are cleaved at different points near each other.
Startpoint (startsite) refers to the position on DNA corresponding to the first base incorporated into RNA.
Stem is the base-paired segment of a hairpin.
Sticky ends are complementary single strands of DNA that protrude from opposite ends of a duplex or from ends of different duplex molecules; can be generated by staggered cuts in duplex DNA.
Stop codons are the three triplets (UAA, UAG, UGA) which terminate protein synthesis.
Strand displacement is a mode of replication of some viruses in which a new DNA strand grows by displacing the previous (homologous) strand of the duplex.
Streptolydigins inhibit the elongation of transcription by bacterial RNA polymerase.
Stingent replication describes the limitation of single-copy plasmids to replication pari passu with the bacterial chromosome.
Stringent response refers to the ability of a bacterium to shut down synthesis of tRNA and rigosomes in a poor-growth medium.
Structural gene codes for any RNA or protein product other than a regulator.
Supercoiling dexcribes the coiling of a closed duplex DNA in space so that it crosses over its own axis.
Superrepressed means the same as uninducible.
Suppression describes the occurrence of changes that eliminate the effects of a mutation without reversing the original change in DNA.
Suppressor (extragenic) is usually a gene coding a mutant tRNA that reads the mutated codon eigher in the sense of the original condon or to give an acceptable substitute for the original meaning.
Suppressor (intragenic) is a compensating mutation that restores the original reading frame after a frameshift.
Synapsis describes the association of the two pairs of sister chromatids representing homologous chromosomes that occurs at the start of meiosis; resulting structure is called a bivalent.
Synaptonemal complex describes the morphological structure of synapsed chromosomes.
Syntenic genetic loci lie on the same chromosome.

T Cells are lymphocytes of the T (thymic) lineage; may be subdivived into several functional types. They carry TcR (T cell receptor) and are involved in the cell-mediated immune response.
Tm is the abbreviation for melting temperature.
Tandem repeats are multiple copies of the same sequence lying in series.
TATA box is a conserved A•T-rich septamer found about 25 bp bdfore the startpoint of each eukaryotic RNA polymerase II transcription unit; may be involved in positioning the enzyme for correct intitiation.
Telomerase is the ribonucleoprotein enzyme that creates repeating units of one strand at the telomere, by adding individual bases.
Telomere is the natural end of a chromosome; the DNA sequence consists of a simple repeating unit with a protruding single-stranded end that may fold into a hairpin.
Temperature-sensitive mutation creates a gene product that is functional at low temperature but inactive at higher temperature (the reverse relationship is usually called cold- sensitive).
Termainal redundancy describes the repetition of the same sequence at both ends of (for example) a phage genome.
Termination condon is one of three triplet sequences, UAG (amber), UAA (ochre), or UGA that cause termination of protein synthesis; they are also called ‘nonsense’ codons.
terminator is a sequence of DNA. represented at the end of the transcript, that causes RNA polymerase to terminate transcription.
Tertiary structure of a protein describes the organization in space of its polypeptide chain.
Testcross involves crossing an unknown genotype to a recessive homozygote so that the phenotypes of the progeny correspond directly to the chromosomes carried by the parent of unknown genotype.
Thalassemia is diease of red blood cells resulting from lack of either α or βglogin.
Thymine dimmer comprises a chemically crosslinked pair of adjacent thymine residues in DNA, a result of damage induced by ultraviolet irradiation.
Topoisomerase is an enzyme that can change the likning number of DNA (in steps of 1by type I; in steps of 2 by type II).
Topological isomers are molecules of DNA that are identical except for a difference in linking number.
Tracer is a radioactively labeled nucleic acid component included in a reassociation reaction in amounts too small to influence the progress of reaction.
Trailer is a nontranslated sequence at the 3’ end of an mRNA following the termination condon.
Trans configuration of two sites refers to their presence on two different molecules of DNA (chromosomes).
Transcribed spacer is the part of an rRNA transcription unit that is transcribed but discarded during maturation; that is, it does not give rise to part of rRNA.
Transcription is synthesis of RNA on a DNA template.
Transcription unit is the distance between sites of initiation and termination by RNA polymerase; may include more than one gene.
Transduction refers to the transfer of a bacterial gene from one bacterium to another by a phage; a phage carrying host as well as its own genes is station and transfer of eukaryotic cellular sequences by retroviruses.
Transfection of eukaryotic cells is the acquistion of new genetic markers by incorporation of added DNA.
Transformation of bacteria describes the qcquisition of new genetic markers by incorporation of added DNA.
Transformation of eukaryotic cells refers to their conversion to a state of unrestrained growth in culture, resembling or identical with the tumorigenic condition.
Transgenic animals are created by introducing new DNA sequences into the germ line via addition to the egg.
Transit peptide is the short leader sequence cleaved from proteins that are imported into cellular organelles by post-translational passage of the membrane.
Transition is a mutation in which one pyrimidine is substituted by the other or in which one purine is substituted for the other.
Translation is synthesis of protein on the mRNA template.
Translocation of a chromosome describes a rearrangement in which part of chromosome is detached by breakage and then becomes attached to some other chromosome.
Translocaation of a gene refers to the appearance of a new copy at location in the genome elsewhere from the original copy.
Translocation of a protein refers to its movement across a membrane.
Translocation of the rigbosome is its movement one codon along mRNA after the addition of each amino acid to the polypeptide chain.
Transmembrane protein is a component of a membrance; a hydrophobic region of regions of the protein resides in the membrane, and hydrophilic regions are exposed on one or both sides of the membrane.
Transplantation antigen is protein coded by a major histocompatibility locus, presuent on all mammalian cells, involved in interactions between lymphocytes.
Transposaes is the enzyme activity involved in insertion of transposon at a new site.
Transpostion immunity refers to the ability of certain transposons to prevent others of the same type from transposing to the same DNA molecule.
Transposon is a DNA sequence able to insert itself at a new location in the genome (without any sequence relationship with the target locus).
Transpostion refers to the movement of a transposon to a new site in the genome. See also nonreplicative transposition, replicative transposition, and conservative transposition.
Transvection describes the ability of a locus to influence activeity of an allele on the other homoloue only when two chromosomes are synapsed.
Transversion is a mutation in which a purine is replaced by a pyrimidine or vice versa.
True-breeding organisms are homozygous for the trait under consideration.
Twisting number of a DNA is the number of base pairs divided by the number of base pairs per turn of the double helix.

Underwinding of DNA is produced by negative supercoiling (because the double helix is itself coiled in the opposite sense from the intertwining of the strands).
Unequal crossing-over describes a recombination event in which the two recombining sites lie at nonidentical locations in the two parental DNA molecules.
Unidirectional replication refers to the movement of a single replication fork from a given origin.
Uninducible mutants cannot be induced.